Soft-Mode FLCs

Soft-mode FLC (SMFLC) was introduced as an alternative way to switch FLC. In this case, instead of vaiying the azimuthal angle ( ) around the tilt cone, i.e., 6 is constant as in SSFLC, SMFLCs use changes in the tilt (9) while (f) remains constant. As a result, SSFLCs exhibit bistability, but SMFLCs are capable of continuous intensity change. The SMFLCs employ smectic-A phase, and uniform alignment is much easier to obtain for SMFLCs than for SSFLCs, which employ smectic-C.  

In the small angle approximation, the 0 term can be neglected. Choosing (t) = 0, a solution of 6 is as follows  

The dynamic behavior of the SMFLC is governed by equating the viscous torque  

In the low field limit, the term can be neglected and Equation (6.44) has the following solutions  

For high field, the term has to be included and the field-dependent response time becomes more apparent  

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For chiral nematic liquid crystals, the method outlined previously for a planar nematic cell has been shown to be quite effective. For smectic-A the preparation method is similar to that for a homeotropic nematic cell. In this case, however, it helps to have an externally applied field to help maintain the homeotropic alignment as the sample (slowly) cools down from the nematic to the smectic phase. The cell preparation methods for a ferroelectric liquid crystal (FLC), smectic-C for surface stabilized FLC (SSFLC) operation, is more complicated as it involves surface stabi-lization. f On the other hand, smectic-A (Sm-A ) cells for soft-mode FLC (SMFLC) operation are easier to prepare using the methods described above. ... 

FIGURE 21.20. Scheme of the collective and molecular dynamics in FLC The Goldstone-mode corresponds to fluctuations of the phase (phason) and the soft-mode of the amplitude d (amplitudon) of the helical superstructure. The high frequency p-relaxation is assigned to librations of the mesogen around Its long molecular axis. 

Two modes contribute to the value of the averaged polarization (P). The first of them (soft mode) is induced due to the amplitude change of the polarization, i.e., variation of the tilt angle 6. This mode is the most important near the phase transition point of the FLC phase and results in the electroclinic eflFect [19, 20, 36]. The second mode, called the Goldstone mode, is responsible for the variation of the phase of polarization, i.e., the azimuthal director angle (p. 

Characteristic times and relaxation frequencies of FLCs can be measured by plotting the Cole-Cole diagrams (see Chapter 2) [32, 46-49]. The dielectric response of the smectic A, near the phase transition, into smectic C phase is approximately one order of magnitude weaker than the corresponding response of the ferroelectric phase. The only contribution to the smectic A response is made by the soft mode with characteristic relaxation frequency [46-48]... 

Dielectric constants and e were measured for the homogeneous and homeotropic FLC orientation, when slowly cooUng a cell down from the phase transition point in the presence of a high magnetic field [12]. It was obtained e = 4, j = 6, Ae = —2 in the kHz region. A relaxation of the soft mode was observed in s, around 20 kHz, while the Goldstone mode manifested itself in an increase of e below 5 kHz [12] (Fig. 7.8). 

Many relaxation processes influence the dielectric spectra of FLCs. Apart from the usual l.f. and h.f. modes characterizing the reorientations of molecules around their principal axes, the Sm C phase shows at least two collective processes. One collective mode, the Goldstone mode (GM), is associated with the fluctuations of the azimuthal angle (the cone motion) it is observed in Sm C phase at low frequencies and is not an activated process. The second mode, the soft mode (SM), is connected with the tilt fluctuations its critical frequency falls in the kilohertz range, from ca. 50 to ca. 500 kHz. The soft mode shows a decrease of frequency in Sm A phase on approaching the transition Sm A -Sm C, but it survives to the lower temperature phase. In special conditions (e.g., after applying an appropriate strength of the bias field ) yet another collective mode can be observed (domain mode). 

In the side-chain polyacrylate 61 (ng. 61), Pfeiffer et al. [75] measured a value of the soft mode relaxation frequency of about -lO Hz, which is diidinctly lower than in low-molar-mass FLCs (lO -lO Hz). No Goldstone rtrade was observed. These results were discussed in leims of a sharp rise of the rotational viscosity with increasing molecular weight of FLC molecules. 

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